It is known that among ultramicroparticles of semiconductors or metals, nano-sized particles having a particle size smaller than the wavelength of an electron (ca. 10 nm), on which the influence of size finiteness on the movement of electrons increases as a quantum size effect, exhibit a specific physical property different from that of the bulk body, as described in Nikkei Sentan Gijutsu (Nikkei Advanced Technology), Jan. 27, 2003, pages 1-4. Nanosized semiconductor particles of a core/shell structure which are covered with a material different from the core portion of the nanoparticles can be functionalized without varying the size or the shape of core particles or are expected to display a characteristic different from that of the bulk material of the core or the shell, therefore, they are noted as a novel highly active catalyst, as a photo functional material or as a material for optical elements. When the surface of light-emitting nanoparticles is exposed, a number of defects existing on the nanoparticles surface become an emission killer, leading to lowering of emission efficiency. To overcome this, the emission intensity can be enhanced by covering the Nanoparticles with a shelling material exhibiting a band gap greater than the band gap corresponding to the emission wavelength of the Nanoparticles, and thereby form a core/shell structure.
With respect to luminescent nanoparticles, there were proposed, as a non-linear optical material, ultramicroparticles having an insulating layer mainly composed of silicon oxide on the surface of silicon nucleuses. Such a material is regarded as being useful as a high intensity light-emitting material achieving high quantum efficiency, as described in JP-A No. 5-224261 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication.
Phosphor particles having nana-structured crystals coated with a glass component can exhibit excitation emission even at a relatively low voltage, leading to a high luminescence efficiency.
There was disclosed in JP-A No. 2005-120117 a phosphor of a core/shell structure, exhibiting high emission efficiency. This phosphor is comprised of a phosphor core at a particle size of 10 nm or less and including a first doping element forming an acceptor level and a second doping component forming a donor level in a semiconductor containing ZnS as a main first main constituent and a II-VI group compound semiconductor as an optional second constituent, and the core being dispersed in a shelling material exhibiting a band gave greater than the band gap corresponding to the emission wavelength of the phosphor.
Thus, an increased band gap energy can be accomplished by decreasing the core particle size to the nana-size level to exhibit quantum size effect and also by forming a core/shell structure, as described above. However, studies have not been made with respect to an effect of scattering in nanosized semiconductor particles size upon optical properties. Further, there cannot be readily accomplished an industrial scale production of nano-sized semiconductor particles exhibiting a uniform particle size distribution.